1. Field of the Invention
This invention relates generally to piezoelectric units and fabrication methods thereof and, more particularly, to piezoelectric quartz crystal units such as filters having improved electrode configurations or improved piezoelectric quartz crystal substrates or combinations of both including fabrication methods therefor.
2. Description of the Prior Art
The radio frequency spectrum is very broad and ranges from about 15 K hz (15,000 cycles per second) to at least about 200,000 M hz (200,000,000,000 cycles per second). Accordingly, it is extremely critical over this entire range of frequencies that a quartz crystal unit be accurate in order to permit transmission and/or reception of the desired frequency signal. This has become especially important in the present environment due to the crowding and closer spacings between assigned frequencies in the available radio frequency spectrum. Thus, in order to attain the goal of having electronic equipment operate with maximum efficiency and accuracy to permit optimum utilization of the limited available radio frequency spectrum, the highest possible precision and stability is required for the transmitted frequency and maximum selectivity is necessary at the receiver.
Although single side band and suppressed carrier communication systems permit the use of more communication channels in a given frequency band, this only serves to place greater emphasis and need for frequency stability and control of both the transmitter and receiver electronic equipment containing quartz crystal units. Many electronic systems including sophisticated equipment used in government or military applications, navigation and ranging systems, etc., depend upon precise frequency control and very accurate time measurements. Thus, the required frequency control and stability needed in trasmitter/receiver systems depends a great deal upon the accuracy and precision of the quartz crystal units which are used as passive elements in oscillator circuits and in electrical wave filters.
Additionally, very high precision and accuracy is also required of quartz crystal units that are used in time keeping applications, devices or systems where equal intervals of time must be precisely marked off. Other applications, for example, where precision type quartz crystal units are used are in digital thermometers, pressure indicators, accelerometers, etc.
The inharmonic modes generated or excited by a piezoelectric quartz crystal unit are known as the "spurs" (spurious modes) whose frequencies generally lie in a relatively narrow range immediately above the main or harmonic mode. In the design and use of piezoelectric quartz crystal units for oscillator applications, these spurious modes that are excited by the driving source are of less concern than in filter applications.
These inharmonic or spurious modes are, however, a serious problem for piezoelectric quartz crystal high frequency filter applications and a need existed to develop improved piezoelectric quartz crystal under which would significantly minimize the effect of these inharmonic modes. Therefore, by reducing or attenuating these spurious modes, the quartz crystal unit can be excited (through the piezoelectric effect) to vibrate at one of its specified resonance modes with improved attenuation of spurious modes.
In the past, manufacturers of piezoelectric quartz crystals generally fabricated either single electrodes per substrate surface or multiple electrodes per substrate surface using square, rectangular, or substantially circular shaped piezoelectric quartz crystal substrates. These prior art types of piezoelectric quartz crystal units utilized electrodes that were either of a generally square, rectangular, or a circular shaped configuration. For example, prior art showing of a substantially square shaped electrode configuration used for circular type piezoelectric quartz crystal units is shown in U.S. Pat. No. 3,564,463 (See FIG. 1).
An example of a prior art piezoelectric quartz crystal unit which utilized a substantially circular shaped quartz crystal substrate and electrodes that had a substantially circular geometry is shown in U.S. Pat. No. 3,396,327. These prior art piezoelectric quartz crystal units with their generally square, rectangular or circular shaped electrodes were only marginal for suppressing or attenuating the spurious modes or inharmonic overtones in many applications.
U.S. Pat. No. 3,582,866 depicts a monolithic type quartz crystal filter which utilizes four resonators (four pie shaped electrodes) on a circular substrate (see FIG. 3). Two of these resonators, namely 43 and 46, provide the respective input and output resonators. Thus, a single four-pole filter unit is shown by this prior art reference with each of the pie shaped electrodes located in one quadrent of the circular substrate. There is no suggestion as to the use of a single or double pie shaped electrode per substrate surface which would be located and arranged to provide significant attenuation of spurious modes. The only substrates disclosed by this reference are rectangular and circular substrates.
U.S. Pat. No. 3,638,146 discloses a piezoelectric ceramic filter which uses a pair of electrodes on a substrate surface and a single electrode on the back substrate surface. The pair of electrodes is configured as two parts of a semi-circle having a relatively straight side across both parts. For this ceramic type of piezoelectric device using a rectangular substrate and this type of split electrode, an improvement in attenuation of spurious modes was apparently achieved. However, use of this type of electrode arrangement (three electrodes per unit and the configuration of the electrodes) has not been adopted or extensively used by the quartz device manufacturers.
Presently, high quality piezoelectric quartz crystal units are mainly man made due to the difficulty in locating large sources of natural quartz crystals. However, in both instances, the cost of producing very good quality piezoelectric quartz crystal substrates to be used to make quartz crystal units with rigorous spurious attenuation specifications can be very high.
The major cost disadvantage associated with prior art piezoelectric quartz crystal units is that they utilize a great deal of valuable and expensive piezoelectric quartz crystal material. Generally, the prior art piezoelectric quartz crystal units utilize either an entire substantially circular piezoelectric quartz crystal substrate or an entire square shaped or rectangular shaped piezoelectric quartz crystal substrate to provide an individual piezoelectric quartz crystal unit. With the definite need to both reduce costs and to increase the producability of piezoelectric quartz crystal units with rigorous spurious attenuation specifications, it became extremely important to develop piezoelectric quartz crystal products that would utilize less piezoelectric quartz crystal substrate material and yet perform significantly better in operation within various electronic apparatus.
Accordingly, a need existed to provide improved piezoelectric quartz crystal units that (a) would have high electrical parameters when used in various applications, (b) further improve the attenuation or suppression of inharmonic modes, and (c) would be relatively inexpensive to fabricate.